I managed to underclock my autotune 13TH S9 by first upgrading the firmware to the 550MHz fixed frequency firmware.

Thereafter ssh into the miner. edit /config/bmminer.conf and change

Maybe one can also undervolt by playing around with


Also one can change the directions of both the fans and the miner should reduce fan speed. Do not change the directions before underclocking though, as the miner will overheat.